Wireless Networks

1
Wireless Networks

• CSE 3461 Introduction to Computer Networking
• Reading 6.16.3, Kurose and Ross

2
Wireless Networks

• Background
• Number of wireless (mobile) phone subscribers now
  exceeds number of wired phone subscribers
  (5-to-1)!
• Number of wireless Internet-connected devices
  equals number of wireline Internet-connected
  devices
• Laptops, Internet-enabled phones promise anytime
  untethered Internet access
• Key idea Wireless communication over wireless
  link

3
Outline

• Introduction
• Wireless links, characteristics
• CDMA
• IEEE 802.11 wireless LANs (Wi-Fi)

4
Elements of a Wireless Network (1)
5
Elements of a Wireless Network (2)

• Wireless Hosts
• Laptop, smartphone
• Run applications
• May be stationary (non-mobile) or mobile
• Wireless does not always mean mobility

6
Elements of a Wireless Network (3)

• Base Station
• Typically connected to wired network
• Relay - responsible for sending packets between
  wired network and wireless host(s) in its area
• e.g., cell towers, 802.11 access points

7
Elements of a Wireless Network (4)

• Wireless link
• Typically used to connect mobile(s) to base
  station
• Also used as backbone link
• Multiple access protocol coordinates link access
• Various data rates, transmission distance

8
Elements of a Wireless Network (5)
9
Elements of a Wireless Network (6)

• Ad hoc Mode
• No base stations
• Nodes can only transmit to other nodes within
  link coverage
• Nodes organize themselves into a network route
  among themselves

10
Characteristics of Selected Wireless Links
200
802.11n
54
802.11a,g
802.11a,g point-to-point
5-11
802.11b
4G LTE /WiMAX
4
3G UMTS/WCDMA-HSPDA, CDMA2000-1xEV-DO
Data rate (Mbps)
1
802.15
.384
2.5G UMTS/WCDMA, CDMA2000
.056
2G IS-95, CDMA, GSM
Indoor 1030 m
Outdoor 50200 m
Mid-range outdoor 200 m 4 km
Long-range outdoor 5Km 20 Km
11
Wireless Network Taxonomy
Single Hop Multiple Hops
Infrastructure (e.g., APs) Host connects to base station (WiFi, WiMAX, cellular), which connects to larger Internet Host may have to relay through several wireless nodes to connect to larger Internet mesh net
No Infrastructure No base station, no connection to larger Internet (Bluetooth, ad hoc nets) No base station, no connection to larger Internet. May have to relay to reach another given wireless node (MANET, VANET)
12
Outline

• Introduction
• Wireless links, characteristics
• CDMA
• IEEE 802.11 wireless LANs (Wi-Fi)

13
Wireless Link Characteristics (1)

• Important differences from wired link
• Decreased signal strength Radio signal
  attenuates as it propagates through matter (path
  loss)
• Interference from other sources Standardized
  wireless network frequencies (e.g., 2.4 GHz)
  shared by other devices (e.g., phones) devices
  (motors, microwaves, etc.) interfere as well
• Multipath propagation Radio signal reflects off
  objects ground, arriving ad destination at
  slightly different times
• . make communication across (even a point to
  point) wireless link much more difficult

14
Wireless Link Characteristics (2)

• SNR signal-to-noise ratio
• Larger SNR ? easier to extract signal from noise
  (good thing)
• BER bit error rate
• SNR versus BER tradeoffs
• Given physical layer increase power ? increase
  SNR ? decrease BER
• Given SNR choose physical layer that meets BER
  requirement, giving highest throughput
• SNR may change with mobility dynamically adapt
  physical layer (modulation technique, data rate)
• Details
• QAM quadrature amplitude modulation
• BPSK binary phase shift keying

10-1
10-2
10-3
10-4
BER
10-5
10-6
10-7
10
20
30
40
SNR(dB)
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
15
Wireless Network Characteristics

• Multiple wireless senders and receivers create
  additional problems (beyond multiple access)

A
B
C
C
Cs signal strength
As signal strength
B
A
Space

• Hidden terminal problem
• B, A hear each other
• B, C hear each other
• A, C cannot hear each other means A, C unaware of
  their interference at B

• Signal attenuation
• B, A hear each other
• B, C hear each other
• A, C cannot hear each other interfering at B

16
Code Division Multiple Access (CDMA)

• Unique pseudo-noise code (PN code) assigned to
  each user i.e., code set partitioning
• All users share same frequency, but each user has
  own chip sequence (i.e., PN code) to encode
  data
• Allows multiple users to coexist and transmit
  simultaneously with minimal interference (if
  codes are orthogonal)
• Encoded signal (original data) (chip
  sequence)
• Decoding inner product of encoded signal and
  chip sequence

17
CDMA Encoding/ Decoding
Channel output Zi,m
Zi,m di . cm
Data bits
Sender
Slot 0 channel output
Slot 1 channel output
Code
Slot 1
Slot 0
Received input
Slot 0 channel output
Slot 1 channel output
Code
Receiver
Slot 1
Slot 0
18
CDMA Two-Sender Interference
Channel sums together transmissions by sender 1
and 2
Sender 1
Sender 2
Using same code as sender 1, receiver recovers
sender 1s original data from summed channel data!
19
Outline

• Introduction
• Wireless links, characteristics
• CDMA
• IEEE 802.11 wireless LANs (Wi-Fi)

20
IEEE 802.11 Wireless LAN (WLAN)

• 802.11a
• Freq. 56 GHz
• Data rate up to 54 Mbps
• 802.11g
• Freq. 2.4, 5 GHz
• Data rate up to 54 Mbps
• 802.11n multiple antennas
• 2.4, 5 GHz range
• Data rate up to 200 Mbps
• 802.11ac, 802.11ad new

• 802.11b
• 2.4, 5 GHz unlicensed spectrum
• Data rate up to 11 Mbps
• Direct sequence spread spectrum (DSSS) in
  physical layer
• All hosts use same chip code

• All use CSMA/CA for multiple access
• All have base-station and ad-hoc network versions

21
802.11 WLAN Architecture

• Wireless host communicates with base station
• Base station access point (AP)
• Basic Service Set (BSS) (aka cell) in
  infrastructure mode contains
• Wireless hosts
• Access point (AP) base station
• Ad hoc mode hosts only

Hub, switch or router
BSS 1
BSS 2
22
802.11 Channels and Association

• 802.11b 2.4GHz-2.485GHz spectrum divided into 11
  channels at different frequencies
• AP admin chooses frequency for AP
• Interference possible channel can be same as
  that chosen by neighboring AP!
• Host must associate with an AP
• Scans channels, listening for beacon frames
  containing APs name (SSID) and MAC address
• Selects AP to associate with
• May perform authentication
• Will typically run DHCP to get IP address in APs
  subnet

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802.11 Passive/Active Scanning
BSS 1
BSS 2
AP 2
AP 1
H1

• Passive scanning
• Beacon frames sent from APs
• Association Request frame sent H1 to selected AP
  
• Association Response frame sent from selected AP
  to H1

24
IEEE 802.11 Multiple Access

• Avoid collisions 2 or more nodes transmitting at
  same time
• 802.11 CSMA sense before transmitting
• Dont collide with ongoing transmission by other
  node
• 802.11 no collision detection!
• Difficult to receive (sense collisions) when
  transmitting due to weak received signals
  (fading)
• Cant sense all collisions in any case hidden
  terminal, fading
• Goal avoid collisions CSMA/C(ollision)A(voidance
  )

Space
25
IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 sender
• 1 if sense channel idle for DIFS then
• transmit entire frame (no CD)
• 2 if sense channel busy then
• start random backoff time
• timer counts down while channel idle
• transmit when timer expires
• if no ACK, then increase random backoff interval
  repeat 2
• 802.11 receiver
• - if frame received OK
• return ACK after SIFS (ACK needed due to
  hidden terminal problem)

sender
receiver
26
802.11 Collision Avoidance

• Idea allow sender to reserve channel rather
  than random access of data frames avoid
  collisions of long data frames
• Sender first transmits small request-to-send
  (RTS) packets to BS using CSMA
• RTSs may still collide with each other (but
  theyre short)
• BS broadcasts clear-to-send CTS in response to
  RTS
• CTS heard by all nodes
• Sender transmits data frame
• Sther stations defer transmissions

Avoid data frame collisions completely using
small reservation packets!
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802.11 Collision Avoidance RTS-CTS Exchange
A
B
AP
time
28
802.11 Frame Addressing (1)
Address 4 used only in ad hoc mode
Address 1 MAC address of wireless host or AP to
receive this frame
Address 3 MAC address of router interface to
which AP is attached
Address 2 MAC address of wireless host or AP
transmitting this frame
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802.11 Frame Addressing (2)
H1
R1
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802.11 Frame More Details
Frame seq (for RDT)
Duration of reserved transmission time (RTS/CTS)
Frame type (RTS, CTS, ACK, data)
31
802.11 Mobility within Same Subnet

• H1 remains in same IP subnet IP address can
  remain same
• Switch which AP is associated with H1?
• Self-learning (Ch. 5) switch will see frame from
  H1 and remember which switch port can be used
  to reach H1

BSS 2
H1
BSS 1
32
802.11 Advanced Capabilities (1)

• Rate Adaptation
• Base station, mobile dynamically change
  transmission rate (physical layer modulation
  technique) as mobile moves, SNR varies

10-1
10-2
10-3
BER
10-4
10-5
10-6
10-7
10
20
30
40
SNR(dB)
1. SNR decreases, BER increase as node moves away
from base station
QAM256 (8 Mbps)
QAM16 (4 Mbps)
2. When BER becomes too high, switch to lower
transmission rate but with lower BER
BPSK (1 Mbps)
operating point
33
802.11 Advanced Capabilities (2)

• Power Management
• Node-to-AP I am going to sleep until next
  beacon frame
• AP knows not to transmit frames to this node
• Node wakes up before next beacon frame
• Beacon frame contains list of mobiles with
  AP-to-mobile frames waiting to be sent
• Node will stay awake if AP-to-mobile frames to be
  sent otherwise sleep again until next beacon
  frame

34
802.15 Personal Area Network

• Less than 10 m diameter
• Replacement for cables (mouse, keyboard,
  headphones)
• Ad hoc no infrastructure
• Master/slaves
• Slaves request permission to send (to master)
• Master grants requests
• 802.15 evolved from Bluetooth specification
• 2.4-2.5 GHz radio band
• Data rate up to 721 kbps

radius of coverage
35
Summary

• Introduction
• Wireless links, characteristics
• CDMA
• IEEE 802.11 wireless LANs (Wi-Fi)